1. Field of the Invention
The present invention relates to a toner fluid composition. In another aspect, there is disclosed a process for electrophoretically depositing the particles of the toner fluid to provide coatings or patterns on a substrate. In a further aspect, a method is provided for enhancing the coatings or patterns to make them electrically conductive.
2. Description of the Background Art
Liquid developers or toners are commonly used in electrophoretic development, particularly for applications where high image resolution is desired. Toner fluids are typically comprised of finely ground pigment particles dispersed in an insulating, organic carrier liquid with charge control agents and/or surfactants added to impart electrostatic charge to the particles and provide steric stabilization toward flocculation. When an exposed electrographic film bearing a latent, electrostatic charge image of a polarity opposite to that of the dispersed pigment particles contacts the toner fluid, the particles migrate to and deposit upon the image, a process known as electrophoretic development.
Latent charge images can be formed using a variety of electrographic processes including: (1) light or X-ray exposure of an electrostatically charged photoconductor, (2) electrical discharge resulting from application of a high voltage between stylus and a conductive support covered with a dielectric receptor, (3) ion projection, (4) electron beam printing, or (5) corona discharge through a metallic mask, etc.
Liquid developers or toners must satisfy a number of requirements; in particular, the concentration of excess ions must be very low in order to prevent competition between the charged toner particles and ions of the same polarity for development of the latent electrostatic image. Volume resistivities greater than 10.sup.12 ohm.cm for the pure carrier liquid are normally required; hydrocarbon and halogen-substituted hydrocarbon liquids are commonly employed. Other factors, such as low dielectric constant (less than 3.5), low viscosity and convenient melting point and boiling point of the medium, colloidal particles with high charge-to-mass ratios and mobilities, small particle size, adhesion of the colloidal particles to the substrate, and stability toward flocculation are also important. Typical particle compositions include pigments such as phthalocyanine blue, monolite red R.S., nigrosene, zinc oxide, and carbon black as well as a number of colored resin composites (i.e. latexes). For a more detailed description of liquid toners and the liquid electrophoretic development process, see R. M. Schaffert, "Electrophotography", The Focal Press, New York, pages 562-574 (1974).
Chemical methods of producing colloidal metal dispersions via metal salt reduction are well established, but entail use of aqueous or polar organic media (high conductivity and dielectric constant). In addition, reductive methods neccesarily produce ionic byproducts which further increase the conductivity of the liquid medium. A recent review dealing with the history, preparation, structural features and properties of aqueous colloidal gold dispersions is given by J. Turkevich in J. Gold Bull. 18, 86-91 and 125-131 (1985). Reductive methods of preparing colloidal metal dispersions in alcohol or alcohol/water mixtures are described by H. Hirai, et. al. in J. Macromol. Sci.-Chem. A13, 633-649 and 727-750 (1979).
Metal evaporation techniques for the production of stable colloidal metal dispersions in polar organic liquids without added surfactants have been described by Klabunde, K. et al. in Proc. SPIE-Int. Soc. Opt. Eng. 821, 206-213 (1988); Klabunde, K. et al., Langmuir 2, 259-260 (1986); Klabunde, K. et al. ACS Symposium Series, 333 (High Energy Processes Organomet. Chem.) 246-59 (1987); Klabunde, K. et al., Langmuir 3, 986-992 (1987); and by Kimura et. al. in Bull. Chem. Soc. Jpn. 56, 3578-3584 (1983) and 57, 1683-1684 (1984). Electrophoretic mobility studies have established that dispersed metal particles in these polar media are electrostatically charged. Although the charging mechanism is uncertain, the electrostatic repulsion between particles is thought to play an important role in stabilizing the dispersions. Unlike dispersions prepared by reductive methods, the evaporative techniques produce no ionic byproducts. However, conductivities and dielectric constants of polar organic liquids are outside the useful range for toner fluids. Similarly prepared dispersions in nonpolar hydrocarbon media undergo rapid flocculation and settling.
Ozin and Andrews in J. Phys. Chem., 90, 2929-2938 (1986) disclose colloidal silver dispersions formed in liquid polyolefins such as poly(butadiene), poly(isoprene) and squalene using metal evaporation techniques.
Magnetic ferrofluids consisting of surfactant-stabilized dispersions of colloidal, ferromagnetic metal or metal oxide particles have been prepared in aqueous media as well as in polar and nonpolar organic liquids. A variety of methods, including prolonged grinding, precipitation, thermal or photochemical decomposition of metal carbonyls and metal evaporation techniques have been used in the preparation of these magnetic dispersions. [See N. Buske, et al. Colloids and Surfaces 12, 195-202 (1984); J. Shimoiizaka, et al. Fine Part. Process., Proc. Int. Symp., Somasundaran, P. Ed., AIME: New York 1980, pp. 1310-1394; E. Papirer, et al. J. Colloid and Interface Sci. 1983, 94, 207-219 and 220-228; M. Kilner, et al. IEEE Transactions on Magnetics 1984, 20, 1735-1737; S. R. Hoon, et al. J. Magn. Magn. Mater. 1983, 39, 107-110; U.S. Pat. No. 4,576,725; U.S. Pat. No. 4,599,184; I. Nakatani, et al. J. Magn. Magn. Mater. 65, 261-264 (1987)].
Magnetic developers for xerography have been patented (U.S. Pat. Nos. 4,252,671 and 4,252,672) which consist of colloidal elemental iron particles dispersed in organic liquid media and stabilized by an active or passive polymer which is bound to the particle surface. Use of these materials as developers is based, as in the case of ferrofluids, on the magnetic properties of the dispersed iron particles and their mobility in a magnetic field. A patent covering dispersions of chromium, molybdenum, and tungsten useful for the preparation of supported catalysts or optical recording media is U.S. Pat. No. 4,252,675. In yet another disclosure, U.S. Pat. No. 4,245,026 describes magnetically responsive toner particles comprised of a low density, imbibitive polymer particle impregnated within the pores of the polymer with iron, cobalt, nickel or their respective oxides.